A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
In an immersion apparatus, a liquid is confined to an immersion space by a liquid confinement structure. The immersion space is between a final lens element of a projection system through which the pattern is imaged and the substrate onto which the pattern is transferred or a substrate table on which the substrate is held. The liquid may be confined to the immersion space by a fluid seal. The liquid confinement structure may create or use a flow of gas, for example to help in controlling the flow and/or the position of the liquid in the immersion space. The flow of gas may form the seal to confine the liquid to the immersion space.
A droplet of liquid or a liquid film (reference hereinafter referring to a droplet also encompasses a film; a film being a droplet covering a larger surface area) from the immersion space may splash onto an exposed bottom surface of the final lens element at a random position which is not normally in contact with liquid in the immersion space. Such a droplet can evaporate. If such a droplet evaporates, a cooling thermal load is applied locally on the final lens element. A local cooling load applied at an unpredefined (or random) position on the final lens element can lead to optical errors, such as one or more of aberrations, other non-correctable imaging and focusing errors, which are unstable within a lot of substrates. Consequently the optical performance may be unpredictably inconsistent over a period of use such as a lot of substrates.